Diamond is the hardest substance on earth, and diamond sintered compacts, which are artificially produced using diamond grains as a starting material, are used, for example, for cutting tools and wear-resistant tools. For example, Japanese Patent Publication Nos. S39-20483 and S52-12126 disclose diamond sintered compacts obtained by sintering diamond grains using an iron group metal, for example, cobalt, as the binder. The diamond sintered compacts disclosed in Japanese Patent Publication Nos. S39-20483 and S52-12126 are widely used as the cutting edge material of tools for cutting nonferrous metals, such as Al—Si alloys, due to their resistance to the chipping caused by the cleavability that is a disadvantage of single crystal diamond.
Among these diamond sintered compacts, diamond sintered compacts with a large average diamond grain size, for example, with an average grain size of at least 20 μm but no more than 100 μm, have a high diamond content and thus an excellent wear resistance. On the other hand, diamond sintered compacts with a small average diamond grain size, for example, diamond sintered compacts composed of microfine diamond grains with an average grain size less than 5 μm, have an excellent resistance to chipping. Within this category of diamond sintered compacts, a particularly excellent resistance to chipping is exhibited by diamond sintered compacts constituted of ultrafine diamond grains with an average grain size no greater than 1 μm.
Efforts to improve the properties of diamond sintered compacts were thus limited to either devising improvements in the resistance to chipping by reducing the size of the starting diamond grains or devising improvements in the wear resistance by increasing the size of the starting diamond grains.
The idea of coating a binder on microfine diamond grains was then devised, as disclosed in Japanese Patent 3,327,080. This coating with a binder made it possible to carry out a high-density sintering in which binder pools, voids, and impurities were scarce. This procedure provided a diamond sintered compact with an improved wear resistance, the lack of which is a weak point of microfine-grain diamond sintered compacts, and this diamond sintered compact has been commercialized and has entered into practical application.
However, even with a reduction in binder pools, voids, and impurities, in order to sinter microfine diamond grains to the same high diamond content as the highly wear-resistant coarse-grain diamond sintered compacts, higher temperatures and pressures are required, even within the context of the sintering conditions at which diamond can be produced, in order to cope with the increase in frictional force between grains brought about by the increase in the surface area of the grains. Under these circumstances, abnormal grain growth of the diamond grains readily occurs in the case of ultrafine diamond grains due to their very high activity. A sintered compact containing regions of abnormal grain growth cannot be cut with a wire electric discharge machine (WEDM), and the mechanical strength of the diamond is also reduced. Since abnormal grain growth is unavoidable when sintering is carried out on starting materials of ultrafine diamond grains with a grain size no greater than 1 μm and an iron group metal such as cobalt (Co) or tungsten carbide (WC)—Co, it is quite difficult to obtain a diamond compact sinter having a uniform structure and a grain size no greater than 1 μm in good yields.
As a consequence, ultrafine diamond grains with a size no greater than 1 μm can in actuality not be sintered at the same content as coarse-grain diamond grains having an average grain size of 20 to 30 μm, and diamond sintered compacts composed of ultrafine diamond grains with a size no greater than 1 μm therefore have a wear resistance inferior to that of diamond sintered compacts composed of diamond grains having an average grain size of 20 to 30 μm.
Japanese Patent 3,391,231 discloses that the chipping resistance, the lack of which is a weak point of coarse-grain diamond sintered compacts is improved by mixing diamond grains with an average grain size of 20 to 30 μm with diamond grains with an average grain size of 2 to 4 μm. However, due to the content of diamond grains with an average grain size of 20 to 30 μm, the strength of such diamond sintered compacts is lower than that of diamond sintered compacts composed of ultrafine diamond grains with a size no greater than 1 μm and the reliability is inadequate from a practical standpoint.
Japanese Patent Application Laid-open No. 2005-239472 discloses a diamond sintered compact in which the diamond grains are bonded to each other using a binder comprising Co and the carbide of, for example, an element from group 4, 5, or 6 of the Periodic Table. The disclosed diamond sintered compact has a specific diamond grain size and content and a specific content of, for example, Co, in the binder and contains the carbide in a specific form, in order to obtain a diamond sintered compact having an excellent wear resistance, chipping resistance, impact resistance, and so forth through an inhibition of abnormal grain growth during the sintering process and a strengthening of direct bonding between the diamond grains. However, due to a lack of any device directed to the interface between the diamond sintered compact and the cemented carbide substrate, tensile stresses act on the bonding region between the cemented carbide substrate and the diamond sintered compact and the diamond sintered compact suffers a reduction in strength and/or delamination and as a consequence consistent production has not been possible.
With regard to the residual stresses in diamond sintered compacts, the relationships between diamond sintered compacts with specific dimensions and the residual stresses in the diamond sintered compacts are reported in J. Am. Ceram. Soc., 77 [6] 1562-68 (1994). The residual stresses in diamond sintered compacts were measured and computed as a function of the diameter of the diamond sintered compact, the thickness of the cemented carbide substrate, and the configuration of the cemented carbon substrate. The maximum residual compressive stress obtained as a result was 1.5 GPa. This shows that it will not be possible to stably produce diamond sintered compacts endowed with residual compressive stresses of 1.5 GPa or more using the prior-art structures in which starting materials comprising diamond grains and an iron group metal such as Co or WC—Co are sintered. Moreover, even if this production were possible, large strains would be produced between the cemented carbide substrate and the diamond layer endowed with high residual compressive stress and stable production would not be possible due to delamination at the interface.    Patent Document 1: Japanese Patent Publication No.39-20483    Patent Document 2: Japanese Patent Publication No.52-12126    Patent Document 3: Japanese Patent No.3327080    Patent Document 4: Japanese Patent No.3391231    Patent Document 5: Japanese Patent Publication No.2005-239472    Non-Patent Document 1: J. Am. Ceram. Soc. 77 [6]1562-68(1994)